1. Field of the Invention
The present invention relates to a feeding device provided to an image forming apparatus such as a copying machine, a facsimile, and a printer. Particularly, the present invention relates to a feeding device that attracts a sheet at the top of sheets stacked in a sheet housing unit onto a belt member and feeds the sheet to a subsequent step such as an image forming section by the surface movement of the belt member, and an image forming apparatus provided with the feeding device.
2. Description of the Related Art
In a feeding device provided to an image forming apparatus and the like, it is important to accurately send out one by one among a plurality of sheets such as recording sheets stacked in a sheet housing unit and thus, a separating mechanism that separates the stacked sheets one by one is required. Generally, separating mechanisms employ a frictional separation method by which a sheet sent from a sheet housing unit by a pick-up roller is separated and fed by frictional force. Examples of the frictional separation type separating mechanisms include a combination of a separating roller and a friction pad and a combination of a separating roller and a reverse roller.
As a feeding device provided with a separating mechanism different from the frictional separation type, Japanese Patent Application Laid-open No. 2007-45630 proposes a feeding device that separates and feeds a sheet using a flow of air. The outline of the feeding device that uses the flow of air to separate and feed a sheet will be explained below.
A feeding device 200 illustrated in FIG. 16 is provided with a paper feed tray 6 that is a sheet housing unit where a plurality of sheets of paper P is stacked, which are nearly horizontal, and a suction belt 2 arranged with a suction device 40 inside the suction belt. The feeding device 200 is further provided with a blowing device 1 that blows air as indicated by the arrow A in FIG. 16 towards the vicinity of the leading edge in a conveying direction (arrow D direction in FIG. 16) of a topmost paper P1 positioned at the top of the paper P placed in the paper feed tray 6. Operating the suction device 40 generates a negative pressure to cause the flow of air as indicated by the arrow B in FIG. 16 that acts to attract the paper P on the lower outer surface of the suction belt 2.
In the feeding device 200, the blowing device 1 blows air to send the air between the sheets of paper P to lift the topmost paper P1 up to the height of the suction belt 2. The topmost paper P1 reached the height of the suction belt 2 is attracted on the lower outer surface of the suction belt 2 by the act of the negative pressure generated by the suction device 40. Thereafter, by the surface movement of the suction belt 2 as indicated by the arrow C in FIG. 16, the topmost paper P1 attracted on the suction belt 2 is conveyed towards the direction indicated by the arrow D in FIG. 16 to reach an image forming section that is a subsequent step and then, image forming is performed.
The blowing device 1 not only lifts the topmost paper P1 up to the height where it is attracted on the suction belt 2, but also separates the leading edge portion side of sheets of the paper P by sending the air between the sheets of paper P. The topmost paper P1 of the paper P with the leading edge portion being separated is attracted on the suction belt 2 and conveyed, thereby allowing the topmost paper P1 to be separated from the other paper P and only the topmost paper P1 to be fed.
The feeding device 200 is also provided with a sheet top sensor 3 that detects the height of the top surface of the topmost paper P1 for maintaining a distance h between the top surface of the topmost paper P1 of the paper P in the paper feed tray 6, which decreases in number by feeding, and the bottom surface of the suction belt 2 within a certain range. Based on a detection signal of the sheet top sensor 3, by controlling an elevating mechanism not depicted which moves a bottom plate 61 of the paper feed tray 6 up and down to adjust the height of the bottom plate 61, the feeding device 200 causes the distance h between the top surface of the topmost paper P1 of the paper P placed on the bottom plate 61 and the bottom surface of the suction belt 2 to be within the certain range.
On the downstream side in the conveying direction from the suction belt 2, a pair of carriage rollers 8 is disposed and the pair of carriage rollers 8 further conveys the paper P conveyed by the suction belt 2 reaching between the two rollers towards the downstream side. Furthermore, on the downstream side in the conveying direction from the pair of carriage rollers 8, a feed sensor 9 that detects the passing of the paper P is provided.
The feeding operation of the feeding device 200 illustrated in FIG. 16 will be explained.
When a command to start feeding is received from a control unit of an image forming apparatus body not depicted, as illustrated in FIG. 17, the blowing of the blowing device 1 and the suction of the suction device 40 are started while the suction belt 2 is being stopped. As the blowing device 1 is started to blow air, the air is blown to the leading edge portion of the paper P as indicated by the arrow A in FIG. 17 and the topmost paper P1 of the stacked paper P is lifted. By starting the suction of the suction device 40, a negative pressure is generated as indicated by the arrow B in FIG. 17 and the topmost paper P1 lifted is attracted on the suction belt 2.
After an elapse of a given time (for example, 3 seconds) from the start of the blowing by the blowing device 1 and the suction by the suction device 40, while the blowing device 1 and the suction device 40 are in operation, as indicated in FIG. 18, the drive of the suction belt 2 and the pair of carriage rollers 8 is started. The suction belt 2 receives the drive transmitted and starts the surface movement in the arrow C direction indicated in FIG. 18, resulting in the topmost paper P1 attracted on the bottom surface of the suction belt 2 being conveyed towards the downstream side in the conveying direction and reaching the pair of carriage rollers 8. The rotation of the pair of carriage rollers 8 in the arrow G direction indicated in FIG. 18 conveys the topmost paper P1 further towards the downstream side.
As depicted in FIG. 19, when the leading edge of the topmost paper P1 conveyed by the suction belt 2 and the pair of carriage rollers 8 is detected by the feed sensor 9, the drive of the suction belt 2 is stopped. When the drive of the suction belt 2 is stopped while the suction device 40 is in operation, a force to stop conveying acts on a portion of the paper P1, at which the paper P1 is attracted on the suction belt 2. However, in the feeding device 200, the respective members are arranged such that a conveying force given to the paper P1 by the pair of carriage rollers 8 is substantially greater than the force to stop conveying. Therefore, the topmost paper P1 is continued to be conveyed by the pair of carriage rollers 8 while the suction belt 2 is being stopped.
The sheet that subsequently comes to the top of the paper P after the topmost paper P1 is defined as a next topmost paper P2. As indicated in FIGS. 17 to 19, while the topmost paper P1 is attracted on the suction belt 2, the leading edge portion of the next topmost paper P2 flaps below the topmost paper P1 as it is receiving the blowing air from the blowing device 1. Accordingly, the leading edge portion side of the next topmost paper P2 is being separated from the paper P below.
Then, immediately after the trailing edge portion of the topmost paper P1 passes through the suction area of the suction device 40, as depicted in FIG. 20, by the flow of air formed between the blowing device 1 and the suction device 40, the next topmost paper P2 is lifted and attracted on the suction belt 2.
According to a predetermined feeding interval, after an elapse of a given time from the operational timing of the feed sensor 9 detecting the leading edge of the topmost paper P1 as indicated in FIG. 19, the drive of the suction belt 2 is resumed. Consequently, similarly to the topmost paper P1 depicted in FIG. 18, the next topmost paper P2 is conveyed by the suction belt 2 towards the downstream side in the conveying direction reaching the pair of carriage rollers 8, and is further conveyed towards the downstream side by the pair of carriage rollers 8.
While the blowing device 1, the suction device 40, and the pair of carriage rollers 8 are in operation, the drive of the suction belt 2 is controlled on and off. The operations depicted in FIGS. 18 to 20 are repeated, thereby sequentially feeding the paper P one by one towards the image forming section.
The feeding device that separates and feeds the sheet by using the flow of air can feed the sheets faster than the feeding device that separates and feeds the sheet by the frictional separation method. The reasons for this are as follows. In the frictional separation method, because it requires a time to frictionally separate the sheet, it has certain limitations with respect to high linear speed and high productivity. On the other hand, in the feeding device illustrated with reference to FIGS. 16 to 20, right after the previous sheet attracted on the suction belt passes through the suction area, the next sheet is separated from the further next sheet and attracted on the suction belt. Consequently, because it only needs to convey the sheet that is separated by the flow of air, it can deal with the high linear speed and high productivity.
The feeding device that separates and feeds the sheet using the flow of air has an advantage of feeding the sheet fast. This leads to the fact that a large number of sheets are consumed in a short period of time. Accordingly, with a single use of such feeding device, a user may have to replenish the sheets to the sheet housing unit in the feeding device very often.
As a structure to reduce replenishing frequency of the sheets, a feeding structure having feeding devices connected in series can be exemplified. As the structure to connect the feeding devices, Japanese Patent Application Laid-open No. 2009-57155 discloses a structure of connecting the feeding devices of the frictional separation type. In the feeding devices disclosed therein, one feeding device is arranged between an image forming apparatus body and the other feeding device and is structured to pass the sheet fed from the other feeding device through inside of the one feeding device. In the structure disclosed therein, a dedicated feed path is provided for the sheet fed from the other feeding device bypassing a sheet conveying unit that separates and feeds the sheet in the sheet housing unit of the one feeding device disposed on the image forming apparatus body side.
FIG. 21 is a diagram illustrating an example of an exemplary image forming apparatus with a feeding device disposed on the image forming apparatus side that is combined with a structure provided with a dedicated feed path for the sheet fed from other feeding device and a feeding device that separates and feeds the sheet using the flow of air.
An image forming apparatus 10 depicted in FIG. 21 is structured with a first feeding device 200a and a second feeding device 200b connected together with an image forming apparatus 10a. In the image forming apparatus 10, a dedicated external device sheet feed path 13 is provided so as to bypass the suction belt 2 forming the sheet conveying unit that separates and feeds the sheet from the paper feed tray 6 in the first feeding device 200a. 
Consequently, with the structure connecting the feeding devices together, it requires a feed path dedicated for the sheet fed from the feeding device on the upstream side towards the image forming apparatus body to be provided in the feeding device on the downstream side. Providing the dedicated feed path causes an increase in component cost and an increase in space for the dedicated feed path for the feeding device on the downstream side.